The Photoelectric Effect
Aim: To test the predictions on the photoelectric effect of the classical theory of light.
Classical theory analogy:
Light waves are like the waves washing onto a beach with rocks (electrons) embedded in the sand
(metal).
Download the
java applet to use
in the experiment
from
Photoelectric
Effect Java Applet
PART A – INTENSITY
Hypothesis 1. Classical theory predicts that as intensity increases (waves get bigger), the number of
electrons being ejected would increase and hence the photocurrent will increase.
Hypothesis 2. Classical theory predicts that as intensity increases (waves get bigger), the ejected
electrons will have more kinetic energy (or velocity, since KE = ½mv2).
Method:
1. Check the box “Show only highest energy electrons” (this is just to simplify the experiment).
2. In the ‘Options’ menu, check “Control photon number instead of intensity”.
3. Set the “Wavelength” to 150nm.
4. Set “Target” to Sodium.
Notice that in these
5. Set the “Intensity” to 20%.
experiments, no
6. Record the photocurrent.
external voltage is
7. Observe the speed of the electrons.
applied (the battery
8. Repeat steps 5-7 using intensities of 40%, 60%, 80% and 100%.
voltage remains at 0.00
9. Repeat steps 5-8 using the other target metals.
at all times).
Results:
Hypothesis 1. Photocurrent increases with intensity
Photocurrent (?)
Intensity
(%)
sodium
zinc
copper
platinum
calcium
20
40
60
80
100
Copy and paste your results table into Excel and graph it. Paste your graph below.
Conclusion: ______________________________________________________________________________________________________
Hypothesis 2. Kinetic energy of photoelectrons increases with intensity
For sodium metal, did the speed of the electrons increase as the intensity increased? ____________
Was this result repeated for all the other metal targets? _________
Would you consider your results for the KE experiment to be reliable? ____________
Why? _____________________________________________
Conclusion: ______________________________________________________________________________________________________
FREQUENCY – PART B
Hypothesis 1. Classical theory predicts that frequency (how often waves arrive) has no effect on the
number of electrons being ejected if the light (wave) is not intense enough.
Hypothesis 2. Classical theory predicts that frequency (how often waves arrive) has no effect on the
kinetic energy of electrons being ejected if the light (wave) is not intense enough.
Method:
1. Check the box “Show only highest energy electrons” (this is just to simplify the experiment).
2. In the ‘Options’ menu, check “Control photon number instead of intensity”.
3. Set the “Intensity” to 20%.
4. Set “Target” to Sodium.
5. Set the “Wavelength” to 800nm.
6. Record the photocurrent.
7. Observe the speed of the photoelectrons (if any).
8. Repeat steps 5-7 using wavelengths of 700, 550, 400, 250 and 100nm.
9. Repeat steps 5-8 using the other target metals.
Results:
Hypothesis 1. Frequency has no effect on photocurrent
Colour
Frequency
Photocurrent (?)
Wavelength
(Hz)
(nm)
sodium
zinc
copper
platinum
calcium
800
infra red
700
red
550
green
400
violet
250
UV
100
UV
Copy and paste your results table into Excel and graph photocurrent vs frequency. Paste your graph
below.
Conclusion: ______________________________________________________________________________________________________
Hypothesis 2. Frequency has no effect on kinetic energy.
For sodium metal, did the speed of the electrons increase as the frequency increased? ____________
Was this result repeated for all the other metal targets? _________
Would you consider your results for the KE experiment to be accurate? ____________
Why? _____________________________________________
Conclusion: ______________________________________________________________________________________________________
SUMMARY
Summarise your results into the table below.
Experiment
Photocurrent
Wave theory
Experimental
prediction
observation
Increase intensity
Increase frequency
Print out this experiment and paste it into your notes.
KE
Wave theory
prediction
Experimental
observation